Conventionally, there is a well-known capacitance type pressure sensor detecting a change in a sensor diaphragm bending during reception of the pressure of a measurement target fluid as a change in capacitance. For example, a capacitance type pressure sensor is used to measure the pressure of a vacuum state in a thin film forming process in a semiconductor manufacturing apparatus or the like and the capacitance type pressure sensor used to measure the pressure of a vacuum state is referred to as a diaphragm vacuum gauge.
The diaphragm vacuum gauge includes a housing having an inlet portion for a measurement target fluid, a sensor diaphragm (provided in the housing) bending during reception of the pressure of the measurement target fluid introduced through the inlet portion of the housing, a sensor unit detecting a change in the bent sensor diaphragm as a change in capacitance, converting the change in capacitance to a voltage value, and outputting the voltage value, and a sensor case covering the housing accommodating the sensor unit therein (see PTL 1, for example). Here, the sensor diaphragm bends and deforms when the surface thereof receives the pressure of the measurement target fluid introduced through the inlet portion of the housing.
In such a diaphragm vacuum gauge, the same material as in the thin film to be processed, by-product material thereof, and the like are accumulated on the surface of the sensor diaphragm. The material accumulated on the surface of the sensor diaphragm is referred to as sediment. When sediment is accumulated on the surface of the diaphragm, an internal stress such as a compression stress or tensile stress is generated according to the state of the sediment.
The generation of the stress pulls or presses the surface of the sensor diaphragm in contact with the measurement target fluid and the force in the thickness direction of the sensor diaphragm becomes unbalanced. This dents or protrudes the surface of sensor diaphragm close to the measurement target fluid.
When the sensor diaphragm bends due to accumulation of sediment on the sensor diaphragm, even though there is no pressure difference between the surface of the sensor diaphragm close to the measurement target fluid and the surface away from the measurement target fluid, detection is performed as if there were a pressure difference, thereby causing zero-point error. Such phenomenon is referred to as a zero-point shift. Since occurrence of a zero-point shift introduces error in pressure measurement by the diaphragm vacuum gauge, the diaphragm vacuum gauge needs to be replaced (see PTL 2, for example).